Development of the Th2 Immune Response Models to Evaluate Allergenicity of Milk Proteins
Food allergy, defined as an adverse immune response to food, is increasing in prevalence. It can be broadly separated into phases of sensitization, in which allergy-triggering Immunoglobulin E (IgE) is generated, and the post-sensitization allergic response, in which the allergic response is triggered by sensitizing allergen. While much is known about the specific mediators that cause allergies, the immune processes that underlie disease progression are less clear. This project has employed mouse models of Th2 immunity to clarify the factors involved in the initiation and maintenance of allergic disease. At the centre of allergic disease is the Interleukin (IL)-4-producing CD4+ T helper type 2 (Th2) cell. One of the key inducers of Th2 cell development in vitro is IL-4, but its involvement in Th2 cell development in vivo is controversial. In our studies, we saw that Th2 cell development could be initiated in vivo by primary, adjuvant-free allergen immunisation in the absence of IL-4. However, Th2 cells were more frequent in IL-4-sufficient conditions. We also determined that genetic lesions that result in loss of one, or both, IL-4 alleles impaired the Th2 cell-mediated allergic process, such that IL-4-heterozygous mice can be considered haplo-insufficient for IL-4 in allergic disease contexts. In addition to the generation of IgE antibody, Th2 cells are implicated in the post-sensitization phase of allergy. Multiple oral challenges of sensitized mice induces elevations in Th2-associated cytokines and elevates intestinal mast cell frequencies. It was the second aim of this project to clarify the role of CD4+ T cells in the post-sensitization intestinal allergic process. We demonstrate a key role for CD4+ T cells in this jejunal mast cell recruitment, and identify that this is required in addition to their established contribution to IgE production. Our investigations also reveal a previously unappreciated role for the CD4+ T cell-derived cytokine IL-3 in oral food allergy. These findings suggest that intestinally localised mast cell-inducer Th2 (Th2m) cells are required for allergic responses generated in the intestine. We also investigated whether specific components of ruminant milks influence the allergic process. While goat and cow milks share significant protein homology, goat milk has lower sensitizing and response-evoking capacity, or allergenicity, than cow milk, in numerous experimental systems. In this project, we compared dominant allergens purified from cow and goat milks for their ability to initiate Th2 cell development. We also examined the ability of one of these allergens to initiate the intestinal allergic process. In these studies, we observed similar Th2 cell development and intestinal mast cell activity in response to both cow and goat milk proteins. These responses indicate that the intrinsic allergenicity of the proteins analysed is not sufficient to explain the differential allergenicity attributed to cow and goat milk. These studies examine the endogenous and exogenous factors that contribute to the development of allergic disease. This project clarifies the role of IL-4 in in vivo Th2 cell development, identifies functional segregation of CD4+ Th2 cells in the intestinal allergic process and further illustrates some of the similarities in the allergenicity of isolated cow and goat milk proteins. Collectively, these studies uncover fundamental aspects of the allergic process which may be useful targets for disease intervention in both prophylactic and therapeutic settings.